1. Field of the Invention
The present invention relates to a brake system provided with: a master cylinder for generating brake-fluid pressure based on a braking operation of a driver; and a hydraulic pressure generator which generates brake-fluid pressure by use of an electrically-controllable actuator.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2003-137084 discloses what is known as a Brake-by-Wire (BBW) brake system. In the BBW brake system, a braking operation of a driver is converted into an electric signal to actuate a device for electrically generating hydraulic pressure, and the brake-fluid pressure generated by this device actuates the wheel cylinders.
In the BBW brake system of this kind, when a failure of the device for electrically generating hydraulic pressure occurs, the fail-safe function is activated, in which the brake-fluid pressure generated by a master cylinder is transmitted directly to the wheel cylinders, thereby braking the wheels. In addition, the BBW brake system employs a tandem type master cylinder, which supplies the brake-fluid pressure individually to two separate hydraulic systems. Thus, even when leakage occurs in one of the two hydraulic systems, the other hydraulic system can brake some of the wheels.
FIG. 4 shows a relevant portion of the structure of a conventional device for electrically generating hydraulic pressure (a motor cylinder 123). Inside a cylinder main body 136, a rear piston 138A and a front piston 138B are disposed free to slide, with a pair of return springs 137A and 137B provided to bias the respective rear piston 138A and the front piston 138B in the backward direction. A rear fluid chamber 139A is formed at the front of the rear piston 138A while a front fluid chamber 139B is formed at the front of the front piston 138B.
A rear reservoir chamber 138a is formed in the outer circumferential surface of the rear piston 138A for the purpose of preventing the air from entering the rear fluid chamber 139A, and a front reservoir chamber 138b is formed in the outer circumferential surface of the front piston 138B for the purpose of preventing the air from entering the front fluid chamber 139B. A rear inlet port 140A of the rear fluid chamber 139A and a rear supply port 149A of the rear reservoir chamber 138a are connected to the master cylinder, and a rear outlet port 141A of the rear fluid chamber 139A is connected to wheel cylinders. In addition, a front inlet port 140B of the front fluid chamber 139B and a front supply port 149B of the front reservoir chamber 138b are connected to the master cylinder. A front outlet port 141B of the front fluid chamber 139B is connected to the other wheel cylinders.
A first rear cup seal C1 is provided to the front end portion of the rear piston 138A so as to face forward (i.e., so as to produce its sealing effects when the rear piston 138A moves forward), and a second rear cup seal C2 is provided to the rear end portion of the rear piston 138A so as to face forward. A first front cup seal C3 is provided to the front end portion of the front piston 138B so as to face forward, and a second front cup seal C4 is provided to the rear end portion of the front piston 138B so as to face backward (i.e., so as to produce its sealing effects when the front piston 138B moves backward).
Assume a hypothetical case where a failure of the conventional motor cylinder 123 occurs, and the wheel cylinders are to be actuated by the brake-fluid pressure generated by the master cylinder. Here, in the above-described structure of the motor cylinder 123, the front supply port 149B is connected to the master cylinder. Accordingly, if a failure of the first hydraulic system occurs, the rear fluid chamber 139A of the motor cylinder 123 comes to be open to the atmosphere. This in turn causes leakage of the brake-fluid pressure generated by the master cylinder, in the course from the front supply port 149B to the rear fluid chamber 139A via the front reservoir chamber 138b and the second front cup seal C4, possibly causing a failure of the second hydraulic system linked to the front fluid chamber 139B.